Bath for anodic treatment of aluminum



Patente Jan. 5, 1937 BATH FOB. ANODIC TREATMENT OF Robert W. Buzzard, Kensington, Md.

No Drawing. Application May 28, 1934,

Serial No. 727,839

6 Claims.

(Granted under the act of March a, 1883, as

amended April 30,

This invention relates to a bath for the anodic treatment of aluminum and its alloys.

The object of this invention is to provide an electrolytic solution for the purpose specified that will permit of use of a large range of voltage and temperature conditions and that contains the methods of treatment such as hot water, or degreasing solvents as benzol, etc., or treatment in known alkali and acid combinations such as sodium hydroxide followed by a water rinse; a nitric acid dip and a water rinse in the order named, before introducing the metal as the anode of the bath. It is further known that chromic acid baths do not produce the best corrosion resistant films on alloys containing over 5% copper nor the high silica alloys. I

In the solutions heretofore used for forming oxide coatings on aluminum it has been deemed necessary to have all the chromium placed in the bath in the form of the hexavalent ion as nearly as possible, that is, as chromic acid, and it has been believed that the reduction of such ions in the bath has been the cause of bath failure. It

has been shown that high concentration of the partially reduced chromium ion does not cause bath failure but, on the other hand, has a highly beneficial efiect on the bath in that much better current densities are so produced. I have found that more satisfactory operation is obtained if some of the chromium is in the'trivalent condition. The maximum percentage of total chromium thus far found satisfactory is 30% and the upper limit of the trivalent chromium is of the total chromium, the preferred values being respectively 3% to 15% and 5% to 40%.

The ratio of the trivalent chromium to the hexavalent chromium, the applied voltage and the temperature of the electrolyte may be varied to produce the type of film desired. It is known that-in straight chromic acid baths a good film may be produced at 40 volts with the minimum current density. In these new baths this is not always the case. It has been found that although for a given constant temperature 40 volts invariably produces the lowest current density in the corrosion resistant film range, it is not always true that the corrosion resistance of the film will compare favorably with other film produced in the optimum voltage range. In general, as the concentration of the solution increases, the voltage range is not only lowered but narrowed. Coupled with this is the efiect of lower temperatures producing lower current densities, and such solutions allow the use of lower temperatures. Any concentration of total chromium may be used if properly adjusted but it hasbeen found best to limit the total chromium to 30% by weight of the Water used for the bath.

The bath may be made up of hexavalent chromium and trivalent chromium to give the desired conductivity. Chromates may be added to this bath if deemed desirable, but it has been found more desirable to use other basic baths for the chromate additions because of initial costs.

Adjustment of the cathode current density, as by changing the cathode area, makes it possible to further control the content of trivalent chromium during operation. 1

Specific sets of condition, given as an illustration of they application of this invention, are as follows:

The bath consists of 4.5

- chromic acid and 1 percent by weight of chromic acid which has been reduced by alcohol and acetic acid: A constant voltage of 40 volts should be applied for one hour at 40 C.

A bath consists of 5% chromic acid and 10.2% chromic acetate of the water of the, bath by weight. applied for about one hour or less at a constant temperature between 30 C. and 40 C.

Other proportions that may be employed are: 3 percent chromic acid, 3 per cent chromic acetate; 9 per cent chromic acid, 10 per cent to 20 per cent chromic acetate; also 5 per cent chromic acid, 5 per cent to 11 per cent chromic acetate.

Although it has been found that the bath can be operated at constant current, it is understood that far more desirable results are obtained at constant voltage and the voltage is varied to suit the characteristics of any particular solution.

One of the main advantages of this process is that as it requires no particular combination of voltage cycles and operates .as a constant voltage per cent by weight of A constant voltage of 40 volts should be constant temperature bath, it may be operated may be produced in the bath by electrolysis of by decreasing the cathode current density, that is, by increasing the cathode area. Further, the bath may be made up to the desired total chromium content and the trivalent chromium formed therein by the heretofore discussed chemical process, namely, alcohol reduction. For this purpose the bath may contain originally 0.5 m. chromic acid which may be reduced to give 10 per cent to 40 per cent thereof as trivalent chro- Although the bath may be operated without agitation, far more desirable operating characteristicsmay be developed by use of some method of agitation. For these particular solutions air is recommended. To produce the best electrical characteristics every efiort should be made to bring the bath to as near constant temperature throughout as possible and precautions along this line should be taken to agitate the solution at the anode to prevent heating of the solution around the anode which willcause abnormally high current densities to occur. I

The cathode may be of any material that is relatively inert to the bath, low carbon steel having been found to be very satisfactory. In practice it has been found very desirable to use the tank as the cathode although the tank may be of other material and the cathodes suspended therein.

The specimen may be washed on removal from the bath, or the solution may be allowed to dry thereon, as in some cases ithas been found desirable to further develop the corrosion resistance of the film by allowing the solution of the bath to dry on the film, it having been shown that such solutions are not detrimental to these films in any manner.

The invention described herein may be manuiactured and used by or for the Government of the United States of America for governmental 1. A method of anodically treating aluminum or its alloys, comprising passing an electric current through the material as an anode in a bath containing 3% chromic acid and 3% chromic acetate at a constant voltage of 40 volts at 40 C.

for one hour, said bath being adapted to be practiced as a continuous process.

2. A method of preparing a bath for the anodic treatment of aluminum or its alloys, comprising adding 0.5 m. chromic acetate to a bath containing 0.5 m. chromic acid.

3. A bath for the anodic treatment of aluminum or its alloys, comprising substantially 5% chromic acid by weight and 5% to 11% chromium acetate.

4. A method of anodically treating aluminum orits alloys, comprising using the article to be treated as the anode in a bath containing substantially 5% chromic acid by weight and 5% to 11% chromium acetate and passing therethrough an electric current at a constant potential of from 30 to 40 volts at a constant temperature of between 30-40 C. for from 30 to 60 minutes, said method being adapted to be appliedas a continuous process.

i :7 V i 2 I 2,068,827 I chromic acid before the aluminum to be treated purposes, without the payment om royalty is placed therein. The last named process is accelerated by addition of the sulphate ion and I claim: s

5. A bath for the anodic treatment of aluminum or its alloys, comprising substantially 9% chromic acid by weight and 10%- to 20% chromium acetate.

6. A method of a'nodically treating aluminum or its alloys, comprising using the article to be treated as the anode in a bath containing substantially 9% chromic acid by weight and 10% to 20% chromium acetate and passing therethrough an electric current at a constant potential of from to 40 volts at a constant temperature of between 30-4 0 C. for from 30 to 60 minutes, said method being adapted to be applied as a continuous process.

ROBERT W. BUZZARD. 

